FIG. 1 illustrates a motor vehicle 3. Many such vehicles contain cooling fans, represented by block 6. Two such fans are illustrated in FIGS. 2 and 3. Fan 9 has equally spaced blades. Fan 12 has unequally spaced blades.
In examining these fans, the inventors have observed that, in operation, and especially at the temperatures encountered in the engine compartment of the vehicle 3 in FIG. 1, the fans 9 and 12 experience deformation. The deformation reduces aerodynamic efficiency.
In addition, the fans are designed to produce minimal noise, but the deformation increases the noise. How a fan produces noise can be understood by a simplified example.
Every time a blade of a fan passes an observer, the blade delivers a small pressure pulse. One can easily prove this by listening to a ceiling fan. Every time a blade passes, a small whooshing sound is perceived. The sound is produced by a small pressure pulse.
A ceiling fan is a low-speed fan. In a high-speed fan, such as that represented in FIG. 1, speeds can reach 2400 rpm, and higher. If the fan has five blades, as illustrated in FIGS. 2 and 3, then 12,000 pulses occur per minute (5×2,400), which correspond to about 200 pulses per second (12,000/60).
The sequence of 200 pulses per second resembles roughly a sine wave of about the same frequency. Humans perceive these pulses as a hum or buzz at about 200 Hz.
To reduce the hum or buzz, various approaches have been developed to reduce the size of the pressure pulses produced by the fans in question, and many have been quite successful. However, when the fans deform in operation as described above, the reduction in noise which was previously attained becomes somewhat compromised.
Therefore, the inventors have discovered that certain cooling fans, especially when operating in a high-temperature environment, experience a change in shape which causes a reduction in aerodynamic efficiency and also produces undesirable noise. The inventors have developed strategies for mitigating these undesirable effects.